1. Field of the Invention
The present invention relates to a control device which controls regeneration of an exhaust emission purifier disposed in an exhaust system of an internal combustion engine.
2. Description of Related Art
A diesel particulate filter (DPF) is disposed as an exhaust emission purifier of an exhaust system on an outlet side of an exhaust pipe of a diesel engine representing an internal combustion engine. The filter catches particulate matters existing in an exhaust gas of the engine. A control device of the engine estimates a quantity of the particulate matters caught and deposited in the filter. When the estimated quantity of the particulate matters exceeds a predetermined value, the control device performs a regeneration control to regenerate the filter. More specifically, the control device controls the engine to heighten the temperature of the filter, so that the deposited particulate matters are burned off and removed from the filter.
As a technique for heightening the temperature of the filter, an engine additionally performs a fuel injection (i.e., post injection of fuel) according to a regeneration control at a timing, which is sufficiently retarded from a timing of the compression top dead center, to burn fuel of the post injection in an exhaust system. Alternately, a start timing of fuel injection is retarded to heighten a temperature of an exhaust gas. When the temperature of the diesel particulate filter is heightened for the purpose of regenerating the filter, it is desired to prevent the filter from being excessively risen. Therefore, the fuel injection is controlled in the regeneration of the filter such that the filter is set within a temperature range appropriate for combustion of particulate matters deposited in the filter.
Published Japanese Patent First Publication No. 2003-172185 discloses a technique for regenerating a diesel particulate filter. In this technique, the temperature of an emission gas is detected by an emission gas temperature sensor disposed on a down stream side of the filter, a control device of a diesel engine performs a feedback control so as to adjust the emission gas temperature at a target emission gas temperature. Therefore, the temperature of the filter is indirectly controlled. Further, this Publication has proposed another technique. In this technique, to indirectly control the temperature of the filter, oxygen catalyst is disposed on an upper stream side of the filter, and an exhaust gas temperature sensor is disposed between the oxygen catalyst and the filter. The control device performs a feedback control so as to adjust an exhaust gas temperature detected by the sensor to a target exhaust gas temperature.
However, injection characteristics of a fuel injection valve differ from those of another fuel injection valve, or injection characteristics of a fuel injection valve are changed with time when the valve is operated in an engine for a long time. Therefore, when actual injection characteristics of a fuel injection valve differ from injection characteristics originally designed, there is a high probability that a quantity of fuel injected from the valve for the regeneration control may be shifted from an appropriate value. In this case, particulate matters of the filter are burned by fuel excessively or insufficiently injected, so that the temperature of the filter is undesirably set out of a temperature range appropriate for the regeneration control. Therefore, when the temperature of the filter is controlled according to an emission or exhaust gas temperature detected by a sensor, following problems may be arisen.
When a fuel injection control is changed from a normal operation mode to a filter regeneration mode, a diesel engine is set in transitional operation conditions, and the temperature of the filter is quickly heightened by a combustion of particulate matters of the filter. However, because a heat capacity of the filter is large, it takes a long time until the temperature of an emission gas is changed to the temperature of the filter, so that there is a large time lag between a change in the emission gas temperature and a change in the filter temperature. Therefore, when a control device performs a feedback control according to an emission gas temperature detected on a down stream side of the filter, there is a high probability that the filter temperature may be set out of a temperature range appropriated for the regeneration control.
Further, even when a control device performs a feedback control according to a temperature of an exhaust gas between the oxygen catalyst and the filter on an upper stream side of the filter, the large heat capacity of the filter prevents the control device from controlling the temperature of the filter exposed to the exhaust gas within a desired temperature range.